1. Field of the Invention
This invention relates generally to the mixing of two or more unlike, solid particulate materials to form homogeneous blends thereof.
More particularly, this invention relates to a method for dispersing fine particles and fine particle agglomerates of unlike materials to form homogeneous mixtures and to thereafter stabilize the mixtures while maintaining their homogeneity. The stabilized mixtures may then be further processed by techniques such as molding, extrusion and reaction to form useful mixtures and composites.
2. Description of Related Art
Many industrial processes require the mixing or dispersion of finely divided particles of two or more unlike substances to obtain a homogeneous mixture or composite. Examples of such processes include the compounding of rubber, the manufacture of pigment and filler pastes for paints and plastics, and the production of ceramic composites, particularly those reinforced with fibers such as inorganic whiskers. In most cases, the properties of the resulting products are directly related to the homogeniety of the mix and the degree to which aggregates and agglomerates are broken down to their ultimate particles or to a mono-disperse form.
Finely divided solid particles tend to clump and to form agglomerates due to the interparticle attraction produced by a variety of molecular, electrical and surface effect forces. These attractive forces become increasingly significant as particle size decreases. At particle diameters of a few microns or less it can require accelerations of many times the force of gravity to separate adjacent particles.
Examples of particulate substances commonly used in industry include carbon black, pigments such as titanium oxide and zinc oxide, whiting, vulcanizing agents such as sulfur, salts such as ammonium nitrate and ammonium perchlorate, plastics such as polytetrafluoroethylene extrusion powders, and finely divided metals and metal oxides. These particulate substances typically have a particle size ranging downwardly from a few microns and seldom can be obtained or maintained in a mono-disperse form. Instead, reliance is placed upon physical working of the materials to obtain acceptable degrees of product homogeneity.
The dispersal or mixing of finely divided materials including fine particle agglomerates ordinarily requires far more involved techniques than simple blending or mixing in order to break down the agglomerates to ultimate particles and disperse them relatively uniformly throughout the mix. Mixing devices employing high velocity impellers and the like are not generally effective. It is usually necessary to employ equipment such as mixers with intermeshing blades or mixers having small clearances between the blades and the body to produce slow speed shear at a high level of mixing power input in order to obtain an intimate and uniform mix. Muller mixers, pug mills, kneaders and other intensive mixers such as Banbury mixers are commonly employed for these purposes.
These common approaches to obtaining homogeneous mixtures of fine particle agglomerates have many practical drawbacks. The work input necessary to obtain the desired degree of dispersion produces intense localized heating. That heating, especially when coupled with high physical shear, results in the degradation of rubbers and plastics with a significant reduction in the average molecular weight It can also result in the decomposition of heat sensitive materials such as certain salts, sometimes with catastrophic effect. Brittle reinforcing fibers cannot be effectively used as the shearing action substantially reduces the average aspect ratio of the fibers When the mix includes reactive components and catalysts, as is the case when compounding many filled rubber and plastics formulations, the localized heating can cause premature and nonuniform polymerization and other reactions. Further, processing is normally done on a batch basis because of the holdup time in the mixer itself makes continuous processing difficult or impossible.
It has been proposed in the prior art to compound certain types of materials at low temperatures using refrigerants such as carbon dioxide and cryogens such as liquid nitrogen. One such prior art technique is disclosed in commonly assigned U.S. Pat. No. 4,474,905 which sets out a process for mixing chemically reactive liquids with particulate solids to form a homogeneous blend. The liquid is chilled to a temperature below its solidification point and is formed into finely divided, solidly frozen particulates and the solid particulates are chilled to a temperature below the solidification temperature of the liquid The materials are then mixed together, without allowing the temperature to rise above the liquid solidification point, using conventional solid-solid or liquid-solid mixers.
Another approach to the low temperature processing of particulate mixtures is shown by Canadian Patent No. 762,437 That patient describes a method for preparing multi-component, spontaneously reactive, chemical formulations in a manner such that they can be maintained in a stable form for long periods of time until it is desired to permit the reaction to proceed. Typical of such formulations are any of a number of epoxy resins in which the resin and hardener are frozen and ground separately in a refrigerant. The two separate components are then intimately mixed in liquid nitrogen and can be stored in that state for an indefinite period of time without reaction. When the liquid nitrogen is permitted to evaporate and the components allowed to warm, melting and reaction occurs. In a system of the sort described in the Canadian patent, segregation of the components during storage in the liquid nitrogen can be avoided only if the specific gravities, or densities, of the various solid components are essentially the same.
An early French patent, No. 945,337 to Potau, describes a process for preparing rubber formulations at low temperatures.
Rubber is cooled until it becomes brittle, at about -80.degree. C., and is crushed to form a powder. The rubber powder is mixed with other ingredients and the mixture is heated under pressure to obtain a final product.
Yet another approach to the low temperature blending of particulate solids is described in copending, commonly assigned, U.S. patent application Ser. No. 790,374.
All of the above described processes have significant disadvantages when used for the preparation of homogeneous mixtures of finely divided particles of two or more unlike materials in which one or more of the materials comprises fine particle agglomerates.